The present inventions relates to imaging systems which include LED printbars comprised of light emitting elements and gradient index lenses.
Light transmitters comprised of bundled gradient index optical fibers or rods are known in the art. For example, U.S. Pat. No. 3,658,407, issued Apr. 25, 1972 to Ichiro Kitano et al., and entitlted xe2x80x9cImage Transmitter Formed of a Plurality of Graded Index Figers in Bundled Configuration,xe2x80x9d describes a light conducting rod made of glass or synthetic resin which has a cross sectional refractive index distribution that varies parabolically outward from a center portion. The rods act as focusing lenses for light introduced at one end. Such lenses are produced under the trade name xe2x80x9cSELFOC;xe2x80x9d a name which is owned by Nippon Sheet Glass Company, Ltd. Relevant optical characteristics of gradient index lens arrays are described in an article entitled xe2x80x9cOptical properties of GRIN fiber lens arrays: dependence on fiber lengthxe2x80x9d, by William Lama, Applied Optics, Aug. 1, 1982, Vol. 21, No. 15, pages 2739-2746.
Gradient index lens arrays are useful in document imaging systems. For example, they are frequently used in the imaging systems of electrostatographic printers which use LED print bars. In that application the gradient index lenses are disposed between the light elements of the LED print bar and the photoreceptor surface so as to focus the light from the light elements into light spots on the photoreceptor surface.
In most imaging applications it is important that the gradient index lenses have an adequate depth of focus. Otherwise, small changes in the relative positions of the gradient index lenses and the surface of interest will cause relatively large changes of the object at the image point. Indeed, it is usually desirable that the depth of focus of a gradient index array be as large as possible while meeting the radiometric efficiency requirements.
FIG. 1 is useful in explaining several important concepts. The illustrated conventional lens L1 has an exit pupil diameter D1, a focal length FL, and a depth of focus DOF. The f/# of lens L1 is the focal length FL divided by the diameter of the exit pupil, or:
f/#=FL÷D1.
As is well known, the depth of focus of a conventional lens can be increased by increasing its relative aperture (or f/#). It is also well known that an increase in the depth of focus results in a reduced radiometric efficiency. Two relationships explain the trade-off of an increase in the depth of focus and a reduction in radiometric efficiency for conventional lenses. First, the radiometric efficiency is inversely proportional to (f/#)2. Second, the depth of focus is directly proportional to the f/#. For example, the depth of focus (DOFxe2x80x2) of the lens L2 of FIG. 2 is greater than that of the depth of focus of the lens L1 since the lens L2 has a smaller exit pupil diameter D2. This is true even though the focal lengths (FL) of lenses L1 and L2 are the same. However, since the radiometric efficiency equals (f/#)2=(D/FL)2, the radiometric efficiency for the lens L2 is less than that of the lens L1. Simply put, while the depth of focus of a lens can be increased by reducing the relative aperture, the price to be paid is a loss in radiometric efficiency. Likewise, radiometric efficiency can be increased, but only with a reduction in the depth of focus.
However, for gradient index lenses it can be shown that the radiometric efficiency is proportional to (noAxc3x97R)2, where no is the axial refractive index of the optical rods, A is a constant which depends upon the gradient index of the lens, and R is the radius of the rods. Additionally, it can be shown that the depth of focus of a gradient index lens is inversely proportional to noAxc3x97R.
Significant to the present invention is the fact that the gradient index lenses currently used in imaging applications usually produce asymmetrical, generally elliptical, spots. Usually, the elongated axis of the spots are aligned in the cross-process direction (the imaged surface moves in the process direction).
U.S. Pat. No. 5,450,157 entitled, xe2x80x9cImaging System Using A Gradient Index Lens Array With Improved Depth of Focus,xe2x80x9d issued on Sep. 12, 1995 to James D. Rees taught an imaging system which uses improved gradient index lenses. Those improved lenses were constructed such that the exit pupil of the lenses were symmetrical and such that the quantity noAxc3x97R is decreased to achieve the radiometric efficiency. While the lenses taught in U.S. Pat. No. 5,450,157 are beneficial, in practical terms they may not be optimal. This is true since the gradient index lenses taught in that patent must be specially formed.
Therefore, a technique which increases the depth of focus of a common gradient index lens while maintaining acceptable imaging radiometric efficiency would be advantageous.
The present invention provides for an improved gradient index lens assembly. That assembly is comprised of an array of asymmetric gradient index lenses for focusing light from an object plane into a focal plane, and a light control film inserted between the object plane and the gradient index lens. The light control film is comprised of microlouvers which block light entering at angles greater than a cut-off angle. The limited angle at which light can enter the gradient index lens increases the effective depth of focus of that lens. Beneficially, the light control film is disposed directly onto the gradient index lens array.